L. R. Linton

Inter- and Intra-Specific Analyses of the Food Niches of Two Sympatric Species of Erpobdellidae (Hirudinoidea) in Alberta, Canada

The feeding of Nephelopsis obscura Verrill and Erpobdella punctata (Leidy) was investigated in Bruce Lake, Alberta, Canada, using specific rabbit antisera against Cladocera/Copepoda, Chironomidae, Oligochaeta, Amphipoda, and Gastropoda. At the species level, prey utilization curves, niche breadth and evenness values for N. obscura and E. punctata showed no significant differences, apart from the absence of Gastropoda in the diet of E. punctata. For any given month Chironomidae, Oligochaeta and Amphipoda dominated the diet of both species; temporal differences in prey utilization however occurred. Both intraand inter-specific resource partitioning occurred as a function of differential weight class utilization of prey. Food niche overlap occurred through the spring, summer and autumn but the probability for interspecific competition was highest in the late summer and early autumn when niche breadth decreased.

A SEROLOGICAL STUDY OF PREY SELECTION BY HELOBDELLA STAGNALIS (HIRUDINOIDEA)

SUMMARY (1) The feeding of Helobdella stagnalis in the field was investigated using specific rabbit antisera against Chironomidae, Copepoda/Cladocera, Oligochaeta, Amphipoda and Gastropoda. (2) Prey utilization was similar in all three field sites, with Chironomidae and Oligochaeta most heavily utilized, Amphipoda and Gastropoda moderately utilized and Copepoda/Cladocera slightly utilized. (3) On any given sampling date all weight classes of Helobdella stagnalis showed the same relative frequency of prey utilization. (4) Prey utilization differed between months with the Chironomidae most heavily utilized in April, and Oligochaeta most heavily utilized in May and June. During other parts of the year there are no significant differences in the pattern of prey utilization. (5) Feeding activity was highest in the spring and summer and lowest in the fall and winter. (6) Annually the 0.0-7.9 mg weight range utilized the Oligochaeta most heavily followed by the Chironomidae. The contribution of the Copepoda/Cladocera to the overall diet was approximately equal to the contribution of the Amphipoda and Gastropoda. The 8.0-20 + mg weight range utilized the Chironomidae most heavily followed by the Oligochaeta. The Copepoda/Cladocera were an insignificant contributor to the overall diet.

Changes in Numerical Dominance and Its Effects on Prey Utilization and Inter-Specific Competition between Erpobdella Punctata and Nephelopsis Obscura (Hirudinoidea): An Assessment

The utilization of prey by Erpobdella punctata (Leidy) and Nephelopsis obscura Verrill was investigated serologically in Cairn Pond when E. punctata was numerically dominant and during the immediate transition period to dominance by N. obscura. At the species level no significant differences in prey utilization were found in either leech species when dominance changed. Intra-specifically E. punctata showed both temporal and differential weight class utilization of prey while N. obscura showed only differential weight class utilization. Niche overlap between E. punctata and N. obscura decreased significantly with the accession of dominance by N. obscura. The evidence for interspecific competition between E. punctata and N. obscura is assessed with respect to the established criteria.

Passive Dispersal of Four Species of Freshwater Leeches (Hirudinoidea) by Ducks

The passive transfer by ducks of two parasitic sanguivorous species (Theromyzon rude and Placobdella papillifera) and two predatory species (Helobdella stagnalis and Nephelopsis obscura) of freshwater leeches was examined experimentally using both adults and cocoons. T. rude adults were transferred both in the nares, while feeding, and also on the ducks body. P. papillifera were transferred at a lower rate on the ducks body only, while H. stagnalis and N. obscura were not transported at all. Ingested adults were not recovered from the faeces of the ducks, but viable N. obscura cocoons were recovered from the faeces of fed ducks. It is concluded that passive transport by birds is probable for the parasitic sanguivorous species and those leeches which attach their cocoons to macrophytes. The evidence does not support the proposal of passive dispersal by ducks for H. stagnalis, a predatory Glossiphoniidae, despite its cosmopolitan distribution.

Chemosensory detection of prey by Nephelopsis obscura (Hirudinoidea: Erpobdellidae)

The chemosensory detection of prey by the freshwater leech Nephelopsis obscura (Erpobdellidae) was investigated in the laboratory. Without direct tactile contact N. obscura of all three size classes tested were unable to detect and react to any of the prey types (Gammarus lacustris, Chironomus sp., Tubifex sp) or prey conditions (live, asphyxiated, homogenate) tested. The length of the starvation period (0–56 days) did not affect these results, nor did it increase activity. N. obscura thus appears to be an opportunistic wanderer rather than a stalker-hunter.

An energetics model of an aquatic predator and its application to life-history optima

SummaryA bioenergetics simulation model of the growth and life history of the aquatic predator Nephelopsis obscura Verrill was developed and validated using both experimentation and sensitivity analysis. Sensitivity analysis demonstrated that the models internal feedbacks resulted in stability similar to homeostatic biological mechanisms. The experimental validation showed the model very accurately predicts growth at 10°C and 15°C but is slightly biased at 20°C. Simulation output was also consistent with the observed data on Nephelopsis from the site from which the simulation input data were obtained and indicated that Nephelopsis growth is more sensitive to prey variation among years than to temperature variation. Although built using data from a population at one extreme of the spectrum observed in life history and growth, the model was able to emulate the growth of Nephelopsis throughout its range. Thus, the variability in size and life history observed in the field can be explained as the result of a plastic phenotype responding to different habitat conditions.

Osmotic and respirometric responses of two species of hirudinoidea to changes in water chemistry

Abstract 1. 1. The differential osmotic and respirometric responses of Erpobdella punctata and Nephelopsis obscura (Hirudinoidea) were investigated as functions of media ionic composition and total dissolved solids (T.D.S.) at both 4 and 15°C. 2. 2. At 4°C neither species showed body weight loss in the range of media tested but at 15°C a 20–27% body weight loss occurred after 20 days. 3. 3. Both E. punctata and N. obscura showed a highly developed ability to regulate water content. 4. 4. N. obscura was a better ionic regulator than E. punctata but no differences between species in respiratory responses were discernable.

Competition and coexistance between Glossiphonia complanata and Helobdella stagnalis (Glossiphoniidae: Hirudinoidea)

SummaryThe prey utilization of Glossiphonia complanata and Helobdella stagnalis from Bruce Lake, Alberta, Canada was examined quantitatively using serological techniques. While considerable prey range overlap occurred between the species, niche overlap was low during the winter, peaked in March and declined through the summer. Temporal differences in feeding and intraspecific weight class differences in prey utilization were found in both species but more distinctly in G. complanata. It is suggested that co-existance of G. complanata and H. stagnalis is based on adjustments of weight class food resource partitioning on a temporal basis.